In the direct synthesis of methylchlorosilanes from silicon and methyl chloride by the Rochow process in the presence of a copper catalyst and optionally, promoter substances, a mixture of the methylchlorosilanes, and a smaller amount of other by-products, are formed. The mechanism and the product spectrum are described, for example, in M. P. Clarke, Journal of Organometallic Chemistry, 376 (1989) 165-222. The influence of the catalyst and the promoters is described, for example, in W. J. Ward et al., Journal of Catalysis, 100 (1986) pages 240-249.
Silicon metal is produced in electrically heated furnaces by reduction of silicon dioxide with carbon, optionally with admixed wood chips. The crude product is brought to the purity required for the preparation of methylchlorosilanes by suitable refining processes. The refined silicon is then usually cast from the refining crucibles into iron troughs, from which it is removed after solidification and prepared by breaking and sieving. It is ground to particle sizes of about 500 .mu.m for use in the direct synthesis.
The requirements on the silicon in respect of chemical composition and particle size distribution for the direct synthesis have been investigated relatively thoroughly. The structural composition and its influence on the reaction with methyl chloride has been the subject of scientific studies only recently. EP-A 350 683 discloses that the structure, which is determined by the cooling process during production of the silicon metal, has an influence on the direct synthesis, because silicon metal produced by atomization gives increased production rates.
The structure of the silicon metal is determined by the size of the crystals of the polycrystalline silicon and the composition and position of the intermetallic phases which precipitate out from the main impurities, for example Al, Ca, Fe and Ti, with silicon in the course of cooling and solidification during the preparation process. The composition of these phases is described, inter alia, in F. Dubrous et al., Electric Furnace Conf. Proc., 1990, pages 241-247, and the ability to influence the formation of these phases is described by A. Schei et al., Proc. Conf. Silicon for Chemical Industry, 1992, pages 11-23. A process for improving the properties by rapid solidification is described by G. Schussler et al., Proc. Conf. Silicon Metal for Chemical Industry, 1992, pages 39-46, and the influence of the crystal structure on the direct synthesis is described by H. Rong et al., Proc. Conf. Silicon Metal for Chemical Industry, 1992, pages 67-83.
It is thus known that the yield and selectivity of the direct synthesis depends on the structure of the silicon, it being possible for the structure to be influenced by choosing the cooling conditions during the solidification process of the silicon and optionally subsequent annealing.